This application is a National Stage filing of an international application under 35 USC 371 of PCT patent application Ser. No. PCT/EP98/06323, filed Oct. 2, 1998, which was published under PCT Article 21(2) in English.
1. Field of the Invention
This invention relates to compositions for sealing discontinuities, for example of the type found in motor vehicles, and to articles used in sealing the discontinuities, as well as to methods of sealing the discontinuities.
2. Description of the Related Art
Motor vehicles such as automobiles and trucks have metal joints and seams that must be sealed. One example is a generally non-planar overlap joint formed by welding together the roof and the side panel of the vehicle to create a square U-shaped trough called a roof ditch. Water collects in the roof ditch and then is drained away from the vehicle.
To prevent water from seeping through the joint it is necessary to seal it. It is often difficult to obtain a good seal, however, because the joint is not planar. In addition, the width of the roof ditch typically varies along its length, further complicating the ability to provide a good seal. Historically, a variety of materials have been used as sealants to fill voids in motor vehicles and exclude dirt, moisture, and other undesirable materials.
Sealants have been supplied as liquid or solid materials depending upon the demands of the application. In the automotive industry, for instance, roof ditch joints are typically sealed using a paste-like plastisol which is then painted, baked and cooled to room temperature.
Hot melt sealants are also used and are generally solid thermoplastic materials which quickly melt with heating and then form a firm bond on cooling. In use, a bead of molten liquid sealant is applied on the joint or seam, much in the way caulking is applied. A worker then brushes or levels the material into a layer of relatively uniform thickness. Applying such a sealant takes skill and often results in a poorly sealed joint or seam. Hot melt sealants cannot be used for visible applications due to non-uniform appearance. A typical hot melt sealant composition utilizes polyolefins that can be difficult to paint and which have poor adhesion to non-porous metallic surfaces, such as steel and aluminum.
Recently there has been a trend towards more xe2x80x9cuser-friendlyxe2x80x9d polyvinyl chloride-based sealants that are provided in the form of a rope or a tape because the handling properties of these materials permit faster installation and eliminate the need to finesse the material after application. Other materials have also been supplied as a strip or a tape.
Once the sealant has been applied, its exposed surface may be covered with a plastic or rubber molding having a flexible top surface, which molding may be painted, for example to match or complement the color of the vehicle exterior. Alternatively, a metal molding may be used. The molding is typically attached to the sealant surface using a mechanical fastener or a pressure sensitive adhesive.
During its life the motor vehicle may be exposed to very cold temperatures of xe2x88x9220xc2x0 C. or lower, for example, xe2x88x9230 to xe2x88x9240xc2x0 C., especially if the vehicle is destined for use in extreme northern climates. Under such conditions, sealants, particularly those that may be used in roof ditch applications, should desirably maintain much of the flexibility that they demonstrate under more ordinary temperatures. This will help prevent the sealant from cracking, breaking, delaminating or lifting up from the surface to which it has been applied as a result of stresses caused by ordinary vehicle use. This not only permits the intrusion of dirt, moisture, and other undesirable materials but can cause a subsequently applied molding to become loose.
In general, this invention relates to compositions for sealing discontinuities and to articles used in sealing the discontinuities, as well as to methods of sealing the discontinuities. The sealant compositions of the invention are especially useful for sealing the types of discontinuities typically found in motor vehicles such as an overlap seam or joint, a butt seam or joint, a depression or indentation, a hole, a spot weld, or a manufacturing defect. The seals are effective in preventing water, dirt, snow, and other undesirable materials from entering the discontinuity and causing corrosion. Quite advantageously, sealant compositions according to the invention have excellent flexibility, even after conditioning at a temperature of xe2x88x9220xc2x0 C.
Accordingly, and in one embodiment, the invention is directed to a composition comprising a curable epoxy-containing material, a first thermoplastic polyurethane component, a curative for the epoxy-containing material, and, optionally, a second thermoplastic polyurethane component that is different than the first thermoplastic polyurethane component. The epoxy-containing material and thermoplastic polyurethane component(s) when melt-blended, but still uncured, display only a single phase. After curing the sealant composition is phase-separated. In addition, the composition passes the test for xe2x80x9cLow Temperature Flexibilityxe2x80x9d that is described more fully hereinbelow.
The thermoplastic polyurethane component is preferably based on a polyether polyol, more preferably, a polytetramethylene oxide polyol. Thus, in another embodiment, the invention relates to a composition that comprises a curable epoxy-containing material, a first thermoplastic polyurethane component that is the polymerization product of a polymerizable mixture comprising a polyisocyanate and a polytetramethylene oxide polyol, a curative for the epoxy-containing material, and optionally, a second thermoplastic polyurethane component that is different than the first thermoplastic polyurethane component. Such compositions provide a melt-flowable sealant for sealing discontinuities in the surface of a substrate, such as those found in motor vehicles.
In certain embodiments the polytetramethylene oxide polyol preferably has a number average molecular weight of at least 600, especially when used in combination with an aliphatic polyisocyanate. In other embodiments, the polytetramethylene oxide polyol preferably has a number average molecular weight of at least 1000, especially when used in combination with an aromatic polyisocyanate. Diisocyanates are preferred for use as the polyisocyanate. Thus, the polymerizable mixture for first thermoplastic polyurethane component preferably comprises either an aliphatic diisocyanate, a polytetramethylene oxide polyol that has a number average molecular weight of at least 600, and a diol chain extending agent, or an aromatic diisocyanate, a polytetramethylene oxide polyol that has a number average molecular weight of at least 1000, and a diol chain extending agent.
The compositions of the invention are preferably tacky at a temperature of about 15 to 25xc2x0 C. In addition, it is preferred that they display at least one glass transition temperature of less than xe2x88x9220xc2x0 C., more preferably, less than xe2x88x9230xc2x0 C., even more preferably less than 40xc2x0 C., and most preferably xe2x88x9240 to xe2x88x9250xc2x0 C. The compositions of the invention further and preferably have a Shore D hardness of less than 50 or a Shore A hardness of less than 85.
The compositions of the invention generally include about 20 to 70 weight percent (wt. %) of the epoxy-containing material, and about 30 to 80 wt. % of any thermoplastic polyurethane component present in the composition, wherein the sum of these materials is 100 wt. %. Preferably, however, the sealant compositions include about 20 to 40 wt. % of the epoxy-containing material, and about 60 to 80 wt. % of any thermoplastic polyurethane component present in the composition. More preferably, the sealant compositions comprise about 20 to 38 wt. % epoxy-containing material, and 62 to 80 wt. % thermoplastic polyurethane component. Even more preferably, the sealant compositions comprise about 25 to 35 wt. % epoxy-containing material, and 65 to 75 wt. % thermoplastic polyurethane component. Most preferably, the sealant compositions comprise about 30 to 33 wt. % epoxy-containing material, and 67 to 70 wt. % thermoplastic polyurethane component.
The sealant compositions may further comprise a second thermoplastic polyurethane component, which preferably is the polymerization product of a polymerizable mixture comprising a polyisocyanate and a polyester polyol. Other materials typically found in the sealant compositions include a thermally activated curative, for example, dicyandiamide, and an accelerator such as an imidazole. The sealant compositions may also include a hydroxyl- or carboxyl-terminated polyester compound. Preferably such materials are semi-crystalline at room temperature with a softening point of less than about 140xc2x0 C., and a number average molecular weight of about 7,500 to 200,000.
Various sealant articles may also be readily provided by combining a layer of a sealant composition according to the invention with another layer that is attached thereto such as a dimensionally stable thermoplastic film or a B-staged, thermosetting plastic cap, though other layers may be used too.
The invention also relates to a method of sealing a discontinuity in the surface of a substrate. This method comprises the steps of:
a) placing over the discontinuity, a sealant composition according to the invention,
b) heating the sealant composition to cause the composition to flow and seal the discontinuity; and
c) allowing the sealant composition to cool.